For the past several weeks I have been losing a great deal of brake fluid but I cannot find the leak. I have removed all four tires to look for wet spots and there are none. There are also no spots under the truck where I park it along with under the hood.

Where else could this be leaking from and what is the best method of finding it?

There are ways it can leak INSIDE itself.
Leaking from front to back of a piston.
The master cylinder is the first thing I think of if there’s no visible leak out. Calipers are another but less common.

No. Leave the brake lines connected to the master cylinder. Just unbolt it from the booster and pull the master cylinder away from the booster enough where you can see if the rear seal is leaking on the master cylinder.

So for the update. There was no fluid inside the cab but I did remove the master cylinder. At first it was hard to pull it out from the booster but once it came out, the fluid started pouring out. It was a lot but it did not seem anywhere close to what I’ve put in over the past few weeks. I assume that there is much fluid still in the booster.

So for the first question, should any fluid be in the booster when I pulled the master cylinder off?

If no, then good, correct? That would mean the master cylinder is leaking and should be replaced. Along those lines, would I need to remove the fluid that did not come out?

If yes, then what?

Regarding the seal on the back of the master cylinder, the locking pin was still in place and of course it was wet with fluid.